For the purpose of this invention sheet material is to be understood as a rolled product having a thickness of not less than 1.3 mm (0.05 inch) and not more than 6.3 mm (0.25 inch), and plate material is to be understood as a rolled product having a thickness of more than 6.3 mm. See also Aluminum Standards and Data, Aluminum Association, Chapter 5 Terminology, 1997.
A cast ingot or slab is a three dimensional object having by definition a length (normally the casting direction in case in (semi)-continuous casting), a width and a thickness, where the width is equal to or greater than the thickness.
The design of commercial aircraft requires different sets of properties for different types of structures of the aircraft. In many parts, resistance to crack propagation either in the form of high fracture toughness or low fatigue crack growth is essential. Therefore, many significant benefits can be realized by improving fracture toughness and fatigue crack growth propagation.
A new material with improved toughness, for example, will have a higher level of damage tolerance. Cyclic loading occurs on a commercial aircraft during the take off/landing when the interior of the aircraft is pressurised. Typically, aircraft may see over 100,000 pressurisation cycles during their normal service lifetime. Thus, it will be noted that great benefit is derived from improved fracture toughness and resistance to fatigue crack growth, both of which are related to cyclic loading.
In the aerospace industry the Aluminum Association alloy AA2024 and modifications thereof have been widely used as a high damage tolerant aluminum alloy, mostly in a T3 temper condition or modifications thereof. As known in the art, alloy of a T3 temper condition has been solution heat treated, cold worked, and naturally aged to a substantially stable condition. Products of these alloys have a relatively high strength to weight ratio and exhibit good fracture toughness, good fatigue properties, and adequate corrosion resistance.
From the European patent no. EP-B-0 473 122 (Alcoa) a method of producing a damage tolerant aluminum alloy sheet product is known, comprising:
(a) providing a body of an aluminum base alloy containing (in wt. %): Cu 3.8-4.5 Mg 1.2-1.85 Mn 0.3-0.78 Fe 0.5 max., preferably 0.12 max. Si 0.5 max., preferably 0.10 max. remainder aluminum, optionally 0.2 max. Zn, 0.2 max. Zr, 0.5 max. Cr, and impurities; (b) hot rolling the body to a slab; (c) heating said slab to above 488.degree. C. to dissolve soluble constituents; (d) hot rolling the slab in a temperature range of 315 to 482.degree. C. to a sheet product; (e) solution heat treating; (f) cooling; and (g) ageing to produce a sheet product having high strength and improved levels of fracture toughness and resistance to fatigue crack growth.
The damage tolerant sheet product obtained by the known method is provided in the T3-condition and is commercially available.